French Pat. No. 2.299.750 in particular and its first Certificate of Addition No. 2.335.079 disclose a device in which a direct monitoring voltage is applied to the terminals of a switch or relay controlled by a transistorised detection circuit. A fault in the insulation or a short circuit creates a leakage current which passes through the transistors and may cut the power to a machine as soon as it reaches a value of the order of 2 mA. A device of this type may be adapted to various types of circuit for monitoring the supply of alternating or direct current, whatever the state of the earth. The detection threshold is adjustable, whereas the detector is connected to the machine circuit for example by means of a diode bridge.
The present invention intends to provide an improved detector, ensuring its own safety, preventing any parasite anomaly introduced by the earth wire and making it possible to connect the neutral directly to earth.